Jose Mourinho said Romelu Lukaku needs a new pair of boots after his drastic loss of form showed no sign of ending during his side's 4-2 Premier League victory at Watford on Tuesday.

The Belgian striker made a dreadful mess of a chance to put United 4-0 ahead, taking an age to shoot with the goal at his mercy before being closed down.

It was the most glaring miss on a night in which he still contributed with some sterling work as United closed the gap on leader Manchester City to five points.

Mourinho was in playful mood when discussing Lukaku, saying the £75 million striker's lack of goals of late could be put down to his lack of a boot endorsement deal.

"I think he needs a big contract with boots because at this moment he doesn't have a contract with any brand, that's why he's playing with black boots," Mourinho told reporters.

"He finished his deal and now he is waiting for the right offer and he is playing in black boots.

"I think he needs a brand to give him the right boots and to pay him the right money so he goes back to goals!"

While Mourinho could afford a smile, thanks to two goals from Ashley Young and one from Anthony Martial that gave his side a 3-0 halftime lead, then a superb fourth from Jesse Lingard after Watford threatened a comeback, Lukaku's struggles are no laughing matter.

Former Manchester United defender Rio Ferdinand believes the returning figure of Zlatan Ibrahimovic may have played a role in Lukaku's drop in form, however.

Ferdinand, now a pundit for BT Sport, said the presence of 36-year-old Ibrahimovic would have mounted the pressure on the young Belgian.

"He'll come out of this," Ferdinand said. "I think that maybe the bigger problem for him is Zlatan Ibrahimovic being there.

"Mentally, having a big striker behind you who wants to play, that's a pressure [coming] from within, and sometimes people find that hard to deal with."

United's tally of 32 points from 14 games is its best since the 2012-13 season, when Alex Ferguson's side went on to win the tile in the Scot's final year at the helm.

Watford, meanwhile, remain eighth in the Premier League, but manager Marco Silva felt aggrieved on Tuesday night as he believed United center back Marcos Rojo should have been sent off.

"We scored the first goal from a penalty and for me in that moment it was a second yellow card for Marcos Rojo, with 11 against 10 it would be really difficult for them," Silva said after the match.

"We made it difficult for them because we scored the second goal. Imagine if it was 11 v 10 at that moment. We started to create more problems for them, but then one counter attack from them and we should do different in that moment."